In the quest for improved semiconductor device performance, semiconductor circuits are becoming denser and the semiconductor devices located therein are becoming smaller. For example, the most common dielectric in MOS field effect transistors has been SiO2. However as the thickness of SiO2 approaches 15 angstroms, substantial problems appear, including, for example, leakage currents through the gate dielectric, concerns about the long-term dielectric reliability, and the difficulty in manufacturing and thickness control.
One solution to the above problem is to use thick (greater than 20 angstroms) films of materials, such as hafnium oxide (HfO2), that have a dielectric constant that is larger than SiO2, e.g., high k gate dielectrics. Thus, the physical thickness of the high k gate dielectric can be large, while the electrical equivalent thickness relative to SiO2 films can be scaled.
Introduction of high k gate dielectrics, such as HfO2, ZrO2 or Al2O3, in gate stacks has proven to reduce leakage current by several orders of magnitude. Such leakage current reduction has enabled the fabrication of complementary metal oxide semiconductor (CMOS) devices with lower power consumption.
It is also desirable to replace Si-containing gates with metal gates that give a workfunction near the band edge for both nMOS and pMOS devices. A number of metal gates are known for nMOS devices; however, metal gates for pMOS devices are rarer.
As such, there is an ongoing need for providing pMOS devices that include a high k gate dielectric in which the workfunction thereof is near the band edge for pMOS devices which does not necessarily have to rely solely on a p-type workfunction metal.